1. Field of the Invention
The present invention relates to a control system for a power supply changeover switch. More specifically, the present invention relates to a control system which is capable of preventing a failure of a power supply changeover switch such as welding of the contacts of the switch.
2. Description of the Related Art
In a power supply system provided with an electric generator and a battery charged by the generator, such as in a power supply system for a vehicle, a power supply changeover switch (hereinafter referred to as "a changeover switch") is used for supplying electric power to a specific large electric load (a load such as an electric heater which consumes a large amount of electric power). In this type of the system, the changeover switch disconnects the battery from the generator and, at the same time, directly connects the large electric load to the generator so that electric power is directly supplied from the generator to the large electric load.
This type of the power supply system is, for example, disclosed in Japanese Unexamined Patent Publication (Kokai) No. 52-111131. The '131 publication discloses a power supply system for a vehicle which supplies electric power to an electric heater of a defogger for removing fog and ice from a windshield of the vehicle. In the system of the '131 publication, the defogger is directly connected to a generator of the vehicle when the defogger is operated, and a battery is disconnected from the generator in order to supply a large electric power to the defogger.
In general, when a battery is connected to a generator, the output voltage of the generator is controlled at a value lower than a predetermined battery charging voltage to prevent an overcharge of the battery. Therefore, if a large electric load such as a defogger and the battery are connected to the generator at the same time, the output voltage of the generator must be controlled at a relatively low value. In this case, since the output voltage of the alternator is low, the electric current and electric power supplied to the electric load also becomes low. This causes insufficient heat generation of the heater and, thereby, the time required for removing fog and ice is prolonged. In the system of the '131 publication, it is possible to set the resistance of the heater of the defogger at a low value to supply a large electric power to the defogger. However, in this case, since the output power of the generator is relatively low due to its low output voltage, the generator cannot supply sufficient electric power to the defogger, and the battery must supply electric power to the defogger in order to make up the shortage of electric power required by the defogger and, thereby, damage to the battery may occur.
In order to solve this problem, the system in the '131 publication disconnects the battery from the generator when the defogger is activated, in order to raise the output voltage of the generator. By raising the output voltage, since the output power of the generator increases accordingly, a sufficient electric power is supplied to the defogger. Further, since the battery is disconnected from the generator, the overcharge of the battery does not occur even though the output voltage of the generator is raised. Namely, in the system of the '131 publication, a large electric power can be supplied to a specific load without increasing the load of the battery.
In the system which selectively connects a battery and a specific load to the generator as disclosed in the '131 publication, a changeover switch is used for switching the connections of the battery and the specific load to the generator. In general, a large capacity mechanical switch (relay switch) is used when a large electric current must be switched. However, since a large electric current must be cut off instantaneously in the changeover switch, welding of the contacts in the switch sometimes occurs. If the welding of the contacts of the changeover switch occurs at the position where it connects the electric load to the generator, for example, the electric load may be damaged due to an excessive power supply and, since the battery is not charged, an excessive discharge of the battery may occur. Further, if welding of the contacts occur at the position of the changeover switch where it connects the battery to the generator, electric power cannot be supplied to the load and, in addition to that, overcharging of the battery may occur when the output voltage of the generator is raised.
It is possible to use a large capacity semiconductor relay switch, in lieu of a mechanical relay switch, for a changeover switch. However, if a large electric current is switched by a semiconductor relay, a dielectric breakdown of the semiconductor may occur, and a similar problem as in the mechanical relay switch may occur.
In general, it is effective to enlarge the contact area of the contacts in the mechanical switch for preventing the welding of the contacts. However, even if the contact area is enlarged, the contacts become liable to welding when the resistance of the contacts increases due to an increase in the roughness of the contact surface after a number of ON/OFF operation of the switch and the same problem may occur. Further, the dielectric breakage can be prevented if a large capacity flywheel diode is connected to the semiconductor relay switch. However, since a large capacity semiconductor relay switch is costly itself and, if a large capacity flywheel diode is required, the manufacturing cost of the changeover switch becomes intolerably high.